Process and apparatus for calibrating a particle counter

ABSTRACT

A process and apparatus for calibrating a particle counter is described. The process comprises the steps of forming a vector gas flow by an aerosol of particles of the same grain size, developing ions in the vector gas with both sign by a bipolar charger, certain particles being electrically charged to a stationary charge state in which the distribution of the number of charges fixed to each particle follows a Gaussian law (Gunn or Boltzmann), passing the charge aerosol into a mobility selector to attract the charged particles to electrodes therein and classify them as a function of the numbers p of their elementary electrical charges e and allowing the electically neutral particles to escape, collecting the neutral particles and passing the neutral particles into the particle counter to be calibrated, the particles counter displaying a value N&#39; o  ; counting the values of N p  and N p+1  of the number of particles of charges pe and (P+1)e fixed by the selector and calculating by the formula ##EQU1## the number N o  representing the number of neutral particles supplied to the counter and comparing N&#39; o  with N p . An apparatus using above process to separately pick-up the charged particles with a view to count the numbers N p  and N p+1  contains a particle counter in which particles of the same grain size are produced comprising a cylindrical case containing an annular cylindrical bipolar charge space and a mobility selector linked with the charge space and are superimposed manner within the cylindrical case.

The present invention relates to methods and apparatuses making itpossible to calibrate an aerosol particle counter, i.e. an apparatusable to determine the concentration (number of particles per volumeunit) of a given aerosol and if possible the grain size spectrum of thesaid particles.

In general terms, various methods are already known and used in industryfor determining the size and/or concentration of particles suspended inthe atmosphere, said methods being performed in the said counters.

One method for calibrating these counters is based on the examination ofa characteristic quantity of particles of an aerosol referred to as itselectrical mobility. This quantity, which defines the varying aptitudeof such a particle to undergo a deflection under the effect of anelectrostatic field can be expressed by the following equation:

    W=ZE

In said vector equation W is the drift velocity acquired by the particleunder the influence of the electrical field E to which it is exposed.The proportionality coefficient Z between the two preceding quantitiesis the electrical mobility in question. This electrical mobility, whichis on the one hand proportional to the electrical charge of the particleand on the other hand is inversely proportional to its grain size, whichis also in accordance with intuition.

There are also equipments called "analysers" or "selectors" ordifferential mobility making it possible with the aid of electrostaticmeans to separate, as a function of their own electrical mobility, thedifferent particles belonging to a given aerosol flux. At present theconcentration or grain size calibration methods of aerosol particlecounters based on the use of such an electrical mobility selector arethe most widely used. Before describing the invention in detail, certaininformation will be given thereon.

With regards to the grain size or dimensional calibration of theparticle counters of an aerosol, the methods used in a completelysatisfactory manner up to now consist either of producing the aerosolfrom calibrated particles, or preparing the aerosol from a solution of asalt in water. In the first case use is e.g. made of latex polystyrenemicrospheres, whose known diameter is between 0.1 and 10 micrometers andthen using a pneumatic machine a liquid suspension of said calibratedparticles is atomized. After atomization and drying, the particle fluxis subdivided into several families, namely:

a) dry residues resulting from the evaporation of droplets not carryinglatex spheres (these are impurities);

b) dry residues resulting from the evaporation of droplets carrying asingle microsphere (singlets);

c) residues resulting from the evaporation of droplets carrying twomicrospheres (doublets), etc.

Thus, there is a discontinuous size particle spectrum constituted byimpurities, singlets, doublets, triplets, etc. In order to obtain astandard flux of given grain size, it is necessary to select aparticular family from those mentioned hereinbefore, which can becarried out without difficulty with the aid of an electrical mobilityselector.

In the second case if the starting aerosol is not constituted by asuspension of precalibrated particles, it can e.g. be an aerosolobtained by the atomization of a solution of a salt in water, a dryresidue is obtained after drying, which is constituted by polydispersedparticles, i.e. whose grain sizes can vary in a random and continuousmanner within a given range. In this case it is merely necessary to passthis aerosol into a differential mobility selector to obtain at itsoutlet quasimonodispersed particles, i.e. all having the same grain sizeand directly usuable for the calibration of a counter.

The invention more particularly relates to the concentrationcalibration, i.e. the number of particles per volume unit. The problemis more difficult and one of the presently used known apparatuses forcarrying out said calibration is described by Jugal K. Agarwal andMichel Pourprix in the article entitled "A continuous flow CNC capableof counting single particles", published in the proceedings of the 9thInternational Conference on Atmospheric Aerosols, Condensation and IceNuclei, Galway, Ireland, 1977. The apparatus and process described inthis article are based on the two following essential characteristics.

Firstly, the aerosol on which working takes place is a monodispersedaerosol, all of whose particles carry a single electrical charge. TheExpert knows how to obtain such a particle population by the methodsdescribed hereinbefore (e.g. calibrated latex spheres from anelectrically mobility selector).

In addition, the sought concentration is measured with the aid of theelectrical current induced by the charged particle flux during thepassage of the particles into an electrometer arranged parallel to thecounter to be tested. The current i measured by this electrometer isexpressed by the equation i=QNe, in which Q is the aerosol flow rate incm³ /s, N is the particle concentration per cm³ and e the elementaryelectrical charge of 1.6.10⁻¹⁹ Coulomb.

Knowing i and Q from the same it is easily possible, at least in theory,to deduce the value N of the concentration of particles per cm³ in theaerosol flow. This known process suffers from a single disadvantage.Thus, it is necessary to assume that the distribution of the aerosolflux between the counter to be calibrated and the measuring electrometerare known.

Moreover, the development of certain new and in particular electronicmethods involving the production of large air volumes with a very highpurity level, i.e. a low particle concentration (white rooms), make itnecessary to be able to estimate concentrations in the atmosphere oftypically 10 to 100 particles per cubic foot, i.e. 10 to 100 particlesin 28,000 cm³. However, the best known calibration methods and inparticular those described hereinbefore do not make it possible tomeasure concentrations below 200 particles/cm³, i.e. approximately5.6.10⁻⁶ particles/cubic foot. Thus, a very important problem is notsolved at present, because the best known calibration methods arelimited to concentrations approximately 10⁴ times higher than thosewhich it would be necessary to reach for carrying out calibrations in ameasuring range compatible with the standards applicable underultra-clean conditions.

The present invention specifically relates to a process for calibratingaerosol particle counters making it possible by using means which aresimple to put into effect to obtain a quasi-absolute measurement of theconcentration of particles of an aerosol and even concentrations such asthose of ultra-clean atmospheres.

The process for calibrating a particle counter according to theinvention is characterized by the following stages:

an aerosol of particles of the same grain size (monodispersed) is formedin a vector gas flow;

said aerosol is exposed to the action of a bipolar charger, e.g.constituted by an ionizing radioactive source able to develop, in thevector gas of the aerosol, ions of two signs which electrically chargethe particles and bring them to a stationary charge state in which thedistribution of the number of charges fixed to each particle follows aGaussian law (Gunn or Boltzmann);

the thus charged aerosol is passed into a mobility selector which, onthe one hand, fixes the charged particles to the electrodes classifyingthem as a function of the number p of their elementary electricalcharges e and, on the other hand, allowing to escape the electricallyneutral particles;

these neutral particles are collected and passed into the particlecounter to be calibrated, which displays a value N'_(o) ;

the numbers N_(p) and N.sub.(p+1) of the particles of charges pe and(p+1)e fixed by the selector are counted and from the same is deducedthe number N_(o) of neutral particles really fed into the counter by theformula ##EQU2## in which ##EQU3## in which d is the diameter of theparticles, K the Boltzmann constant, T the absolute temperature inKelvins and ln the natural logarithm;

N'_(o) and N_(o) are compared.

As can be seen, the above process is essentially based on the twofollowing physical phenomena.

Firstly, the charging laws of aerosols in a bipolar ionized medium leadto a stationary electrical state, in which the distribution of thenumber of charges fixed to each particle follows a Gaussian law(Boltzmann or Gunn law) in the form: ##EQU4## in which e is theelementary electrical charge (4.8·10⁻¹⁰ ues cgs)

d the diameter of the particles (cm)

K the Boltzmann constant (1.38·10⁻¹⁶ erg/^(o))

T is the absolute temperature (^(o) K)

p is the number of electrical charges carried by the particles

p is the mean charge of the aerosol (if p=0, Boltzmann law,

if p≠0, Gunn law)

N_(p) is the number of particles carrying p charges per volume unit(cm⁻³)

Z is the total number of particles (cm⁻³).

Finally, the very particular form of this Gaussian charge distributionlaw makes it possible to link the numbers No of neutral particles of athus charged aerosol and the numbers N_(p) and N_(p+1) of particles ofsaid aerosol having p elementary charges and p+1 elementary charges, inaccordance with the formula: ##EQU5## It is therefore easy by using aparticle electrical mobility selector, to separately detect the chargedparticles with a view to selecting the numbers N_(p) and N_(p+1), todeduce therefrom in accordance with the above formula the number ofneutral particles N_(o) contained in the said aerosol and to feed themto a counter to be tested, in order to finally compare this calculatednumber with the actual counter reading. This arrangement makes itpossible to remove the uncertainty concerning the distribution of theparticles between the counter to be tested and the standard sensor,which is one of the defects of the prior art process. It also makes itpossible to utilize very accurate surface deposit coupling methods.

This method has the advantage of being an absolute method. It isessentially dependent on the accuracy with which it is possible to countN_(p) and N_(p+1), ±5% being the typical uncertainty range. Moreover, itmakes it possible to calibrate counters at ultra-low particleconcentrations, such as are nowadays encountered e.g. in white rooms.Compared with the prior art methods, the invention makes it possible toreduce by a factor of approximately 10⁴ the particle concentrationsnecessary for carrying out the calibration of counters, which are thuscalibrated in their white room use ranges.

Obviously, the mobility selector used in the process according to theinvention can, at least in theory, be of a random nature and can be oneof the selectors most widely used hitherto having a planar longitudinalgeometry or an axial cylindrical geometry.

However, the Applicant has found that a new type of mobility selectorwas able to permit a simpler and more reliable performance of theprocess according to the invention. This mobility selector is anelectrostatic sensor comprising two spaced, parallel, coaxial conductordisks between which is established an electrical field by raising themto different potentials, the space between the two disks communicatingover its entire periphery with the atmosphere to be examined, a centralsuction system being provided in the said space so as to bring about acirculation there, from the periphery of the disks, of part of the saidatmosphere in the form of a stable, centripetal, laminar flow.

In its sijmplest form shown in FIG. 1, sadi electrostatic sensor used asa mobility selector is constructed in the following way. The sensoressentially comprises a flattened, circular, cylindrical case 2,provided along its central axis with an inlet 4 for the injection flowQ_(o) of entraining gas under the action of a pump 6. Along the sameaxis, but in the upper part is provided a discharge pipe 8 for theatmosphere sucked in under the action of the suction pump 10. In theupper part of the case 2 is provided an annular slot 12 for thepenetration of the atmosphere to be examined under the angle of itsaerosol particle content. The sampling flow rate Q1 of the gas to beexamined in order to make it flow in the case 1 results from thedifference between the gas suction flow rate Q2 under the action of thepump 10 and the injection flow rate Q_(o) at the inlet and under theaction of the pump 6.

Within the case 2 is provided a solid, thick disk 14, which may or maynot conduct electricity, on whose upper surface rests one of the twocoaxial conductor disks 16 which, together with the upper part 18 of thecase 2, form the two coaxial, conductor disks characteristic of theinvention. The upper disk 18 is connected to earth or ground, whereasthe lower disk 16 is brought to a high positive or negative voltage withrespect to the said earth or ground. The entraining gas moved by thepump 6 and injected into the pipe 4 penetrates the case 2 by the lowerpart and is distributed therein in accordance with a symmetry ofrevolution symbolized by the arrows F and traverses an annular filter20, which completely purifies the same whilst eliminating all suspendedparticles which it can contain and also regularizes the flow. Once thisgas has been filtered it passes round the upper part of the thick disk14 and gives the atmosphere flow Q1 to be examined a stable,centripetal, laminar flow in the space between the two coaxial,conductor disks 16 and 18. The said gas is then subject to suctionaction in the centre of the apparatus by the pipe 8 under the effect ofthe pump 10. After a certain operating time necessary for theelectrostatic trapping on the disks 16 and 18 of the aerosol particlescontained in the atmosphere flow sampled through the opening 12, it ispossible to open the apparatus and observe the disk 16, which has theappearance represented in FIG. 2 and where it is possible to see theparticles having a given sign, if they are all of the same grain size,deposited in concentric, annular zones corresponding to their differentelectrical mobilities, i.e. to their different electrical charges, 1,2,. . . p, p+1.

Therefore the number of particles of the different consecutive annularzones is related to the values N₁, N₂ . . . N_(p), N_(p+1) referred tohereinbefore and which the Expert knows how to count using a surfacedeposit analyser and per se known data processing.

This type of electrical mobility particle selector makes it possiblethrough its circular symmetry and its stable, centripetal, laminar gasflow to bring about a total collection of the charged particles (whilstregulating the potential difference between the disks 16 and 18 and theflow rate Q₂ for this purpose), as well as an easy counting thereof byannular zones with the aid of known data processing means.

Thus, the counting of particles carrying p and p+1 charges deposited ontwo consecutive, concentric, annular zones permits the precisecalculation of the number N_(o) of neutral particles contained in theexamined aerosol. In practical terms, usually this calculation iscarried out on the basis of numbers N₁ and N₂ of particles carrying oneand two elementary charges, i.e. two first annular zones.

If in theory the calculation of N_(o) from N_(p) and N_(p+1) can becarried out according to the previously given formula, it is more usualto utilize the following formula, which is derived from the firstformula and which gives the value of N_(o) as a function of the numbersof particles C₁ and C₂ respectively carrying one and two elementarycharges, such that they can be counted on the deposit surface and asdeposited throughout the time Δt of the said deposit. This formula iswritten in the following way ##EQU6## with N_(o) being the concentrationof standard neutral particles (particle/cm³), Q_(o) is the extractionflow rate of the electrostatic sensor (cm³ /s), Δt is the time duringwhich the deposit takes place(s), whilst C₁ and C₂ represent the numberof particles carrying one and two charges, counted on the depositsurface and directly related to the previously defined concentrations N₁and N₂.

The Applicant has found that it was possible when the calibrationprocess of a particle counter according to the invention was performedwith the aid of an electrical mobility selector having a circularsymmetry and a centripetal flow, to obtain a particularly simple andcompact calibration apparatus, whose integrated form makes itparticularly easy to use.

This apparatus for performing the calibration process of a particlecounter, in which the particles of the same grain size are produced byselective electrostatic filtration, in a first mobility selector, of aprimary aerosol having several discrete grain sizes is characterized inthat it comprises, in the same cylindrical case and positioned one belowthe other, an annular cylindrical bipolar charging space e.g. having anionizing source and receiving an aerosol of monodispersed particles, anda mobility selector communicating with the charging space and dispersingon its electrodes and as a function of their charges, all the chargedparticles which it receives, the suction flow rate of said mobilityselector only containing the neutral particles to be counted.

Preferably, the apparatus also has an electrical mobility preselectorwith a circular symmetry. As a result of this preselector, at the inletit can receive the primary aerosol having several different grain sizes.Preferably, the apparatus also has a primary aerosol injection chamberlinked with the bipolar charging space either directly, or via theelectrical mobility preselector.

In its preferred embodiment, the calibration apparatus according to theinvention combines in the same cylindrical case and in four superimposedstages the injection functions of the first and second mobilityselectors and the bipolar charging means. The two mobility selectorstages are separated by an ionizing chamber having an e.g. alpharadioactive source, which brings about the bipolar charging of theaerosol particles, which is a characteristic of one of the essentialstages of the calibration process according to the invention.

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and the attached drawings, wherein show:

FIG. 1 the diagram of a charged particle electrical mobility selectorwith circular symmetry and a centripetal laminar flow.

FIG. 2 in plan view the deposits, in the form of successive annularzones, of particles dispersed as a function of their charge on thecollection plate of the apparatus of FIG. 1.

FIGS. 3a to 3d the distribution of the electrical charges of theparticles of an aerosol exposed to the action of a bipolar charger inthe case of a so-called Boltzmann symmetrical distribution.

FIG. 4 the same charges of a particle flux in the case of an equilibriumdiffering from that of Boltzmann and known as the Gunn equilibrium.

FIG. 5 the grain size spectrum of the particles obtained during thepreparation of an aerosol by spraying a solution of precalibrated latexpolystyrene spheres or balls.

FIG. 6 the diagram of an installation for performing the calibrationprocess according to the invention in the general case.

FIG. 7 the diagram of a calibrating apparatus for a particle counteraccording to the invention.

FIG. 8 in greater detail the industrial embodiment of the diagram ofFIG. 7.

FIGS. 1 and 2, which have already been described, illustrate aparticular construction of the electrical mobility selector moreparticularly applicable to the case of the process of the presentinvention.

FIGS. 3a to 3d make it possible to understand the effect of the bipolarcharging phase of the particles of an aerosol with the aid of ions ofboth signs produced in the vector gas of said same aerosol. On theabscissa appears the number of charges by particles and on the ordinatein reduced coordinates the number of particles having a given electricalcharge relative to the total number of particles. FIG. 3a relates toparticles with a diameter of 0.1 micrometer, FIG. 3b to particles of 0.2micrometer, FIG. 3c to particles of 0.5 micrometer and FIG. 3d toparticles of 1 micrometer. The hypothesis illustrated in FIGS. 3a to 3dis that of a Boltzmann distribution in which the aerosol is overallneutral and in which the charged particles of one sign are in equalnumbers compared with those charged with the opposite sign. It ispossible to see in the manner known to the Expert that the larger thediameter of the particles, the less neutral particles there are and themore particles there are carrying charges which are a multiple of theelementary charge e.

The interest of the Boltzmann distribution is that, as has been revealedby the Applicant, the knowledge of the number of particles carrying pcharges and the number of particles carrying p+1 charges, or in otherwords the knowledge of the two points of the preceding "curve" makes itpossible to determine the number of particles N_(o) which are notcharged. In the particular case of the presently described Boltzmannconfiguration, even in theory it is sufficient to e.g. know the numberN₁ of particles having an elementary charge to determine the number ofneutral particles N_(o).

In the general case of a bipolar charging, as described hereinbefore,the equilibrium obtained by the different charges on the particles is anequilibrium in accordance with Gunn's law in the manner shown in FIG. 4and which differs from that referred to hereinbefore by the fact thatthe charge distribution "curve" is no longer symmetrical around the bandcorresponding to the neutral particles N_(o). In both cases theknowledge of two random consecutive values of the number of particlesN_(p) and N_(p+1) makes it possible to determine the number of neutralparticles N_(o), which is the essential condition making it possible toperform the process according to the invention. Thus, it is the bipolarcharging phase of the gas vector of the particles of a given aerosolwhich makes it possible, by applying the preceding characteristics ofthe charge spectra, to determine the exact number N_(o) of neutralparticles contained in a given aerosol flux and therefore perform theprocess of the invention.

On referring to FIG. 5 an explanation will be given of the physicalphenomena which occur when, for calibrating in grain size a particlecounter, an aerosol of standard grain size is produced fromprecalibrated latex polystyrene microspheres. For this purpose, the mostwidely used process consists of suspending in a gas, generally air,monodispersed aerosols, i.e. all of whose particles have the same grainsize.

For this purpose, use is made of latex polystyrene microspheres marketedby several manufacturers and whose particle diameter is generallybetween 0.1 and 10 micrometers with an extremely small standarddeviation of approximately 1%. Firstly a liquid suspension of suchcalibrated particles is formed, generally in ultra-pure water. Thisliquid is then atomized by pneumatic means suspending the droplets inthe vector gas. The aforementioned preparation method obviously leads toan aerosol, whose particles have variable dimensions, because they canhave 0,1,2 or p latex microspheres. After drying these droplets, the gasflow obtained has in suspension several families of particles, namelythose whose concentration is shown in FIG. 5 as a function of theirdiameter.

The different concentration peaks shown in FIG. 5 correspond to thefollowing cases:

Peak a corresponds to dry residues resulting from the evaporation ofdroplets, whereof all that is left are certain impurities, but no latexmicrospheres.

Peak b corresponds to dry residues resulting from the evaporation ofdroplets carrying a single microsphere (these droplets can be calledsinglets).

Peak c corresponds to dry residues resulting from the evaporation ofdroplets carrying two microspheres (which can be called doublets).

Peak d corresponds to triplets, etc.

The different families of particles also carry, as a result of thefriction which has occurred during atomization, a random number ofelectrical charges, but which is obviously equal to a multiple of theelementary electrical charge e.

In accordance with a known procedure, it is merely necessary to passsuch a particle flux into a mobility selector in order to obtain at itsoutlet and obviously as a function of the chosen setting, a flux ofparticles all having the same grain size (monodispersed) and having asingle electrical charge. In other words, this passage into anelectrical mobility selector makes it possible to select the group b ofsinglets of FIG. 5, which all have the elementary electrical charge e.

FIG. 6 shows the diagram of an installation making it possible toperform the process of calibrating a particle counter according to theinvention. It shows a latex microsphere aerosol generator 22 functioningin the manner described relative to FIG. 5. At the outlet from saidaerosol generator 22, the particle flux passes through a dryer 24 andthe part 26 of the aerosol flux which has not been discharged to theoutside at 28 through a filter 30, penetrates a first electricalmobility differential analyser or electrical mobility selector 29. Thelatter is regulated in such a way as to only allow to pass through itsoutlet 31 monodispersed particles, e.g. singlets, all having the sameunitary electrical charge e. This aerosol flux is then passed to abipolar charger 32, which is a space in which an e.g. alpha ionizingradioactive source ionizes the vector gas of the aerosol causing theappearance therein of gas ions of both signs. This ionized vector gas inturn charges the particles of the aerosol which are then in thestationary electrical equilibrium of FIGS. 3 or 4. The thus chargedaerosol flux is then injected into a second differential mobilityselector in the following way.

At the outlet 34 from the bipolar charger 32, the gas flux is injectedinto a sealed enclosure 36 having a pipe 38 for dicharging any gasexcess through the filter 40. In said enclosure 36, which is thereforerapidly entirely filled with the vector gas in the preceding stationaryelectrical equilibrium, is located in the second mobility selector 42and which, as indicated in FIG. 6, can be an electrical mobilityselector with circular symmetry and a centripetal, gas flow. Thismobility selector 42 has two electrodes respectively connected for theexternal electrode 44 to earth and for the internal electrode 46 to ahigh potential of ±V typically of several thousand volts for particleshaving a diameter of approximately 1 micrometer. The mobility selector42 is traversed by an entraining gas entering the same through the inlet48 across the filter 50 and passes out of the same at 52. According tothe invention, the selector 42 has a suction slot 54 by which theaforementioned entraining gas flux sucks the vector gas from the aerosoland makes it pass into the vicinity of the electrode 46. As the latteris charged to a high fixed negative or positive voltage, in the mannershown in FIG. 2, the different charged particles of the aerosol vectorgas are distributed into concentric annular zones, each of whichcorresponding to particles having an integral number of elementaryelectrical charges.

In principle, the mobility selector 42 is regulated so as to fix all theelectrical particles of both signs on the two electrodes 44 and 46 andonly allows the passing out at 52 of the uncharged neutral particles inconcentration N_(o). According to the invention, said neutral particleflux is supplied to the intake 56 of a counter 58 to be calibrated andit is then merely necessary to compare the number N_(o) calculated onthe basis of the aforementioned formula linking N_(p), N.sub.(p+1) andN_(o) with the reading of the counter 58 to be able to carry out anabsolute calibration of said particle counter.

Obviously, for the precise calculation of the number N_(o) of neutralparticles supplied to the particle counter 58 to be tested, as afunction of the case, one or other of the two formulas givenhereinbefore is used, said formulas being equivalent and are directlyderived from one another.

FIG. 7 shows a particularly interesting embodiment of the apparatus ofFIG. 6. The Applicant has found that if the two electrical mobilityselectors 29 and 42 of FIG. 6 were produced in the form of circularselectors with a radial flow, it was possible to concentrate the essenceof the apparatus in a single cylindrical case. Such a case, shown inFIG. 7, has in juxtaposed superimposed manner the first mobilityselector 29, the second mobility selector 42, as well as an injectionchamber 60 and a bipolar charging chamber 62. These different componentscommunicate pairwise with one another, namely the injection chamber 60with the first mobility selector 29 via the annular slot 64, the firstmobility selector 29 and the bipolar charging chamber 62 via the annularslot 66 and the charging chamber 62 and the second electrical mobilityselector 42 via the annular slot 68.

The entraining gas inlets of the two mobility selectors 29 and 42 arerespectively located at 70 and 72 across entraining filters 74 and 76.The primary aerosols from the generator 22 of FIG. 6 are introduced intothe apparatus at 78 and enter the injection chamber 60. The unused partof said primary aerosols is discharged to the outside by the duct 80.The first mobility selector 29 is connected to a suction tube 82, whichdischarges to the outside the gas flow which has entered said sameselector at 70, whereas the suction tube of the second mobility selector42 is located in accordance with the axis of the apparatus at 84 and,according to the invention, only the neutral particles of the aerosolsextracted from the chamber 62 pass out through the said tube.

The annular outlet slot 66 of the first mobility selector 29 is at aradial distance from the axis of the apparatus chosen as a function ofthe grain size and the electrical characteristics of that part of theaerosol which is to be sucked by the slot 66 into the bipolar chargingspace 62. In the manner diagrammatically shown in FIG. 7, said space 62has a radioactive ionizing source 86 for charging with ions of bothsigns in accordance with the Boltzmann or Gunn law the vector gas of theaerosol part which has entered via slot 66 into the charging space 62.

The feed slot 68 of the second electrical mobility selector 42 islocated on the periphery of the latter, so as to permit a flow of theaerosol-charged gas phase along the collecting electrode 88 for theaerosol-charged particles. According to the invention, the electricalvoltage ±V to which is raised said collecting electrode 88 is chosen, asis the suction flow into the pipe 84 on leaving the selector 42, in sucha way as to permit the collection on the electrode disk 88 of all theelectrical charges which have entered the selector 42 and so as to onlyallow the suction through the outlet 84 of the neutral particles, whichare then supplied to the counter 58 to be tested and which is not shownin FIG. 7.

With reference to FIG. 8, an industrial embodiment of the integratedstructure of FIG. 7 will be described. FIG. 8 essentially shows acompact cylindrical block into which are integrated in juxtaposed mannerthe injection chamber 60, the first mobility selector 29, the bipolarcharging chamber 62 and the second electrical mobility selector 42. Itis also possible to see in FIG. 8 the tube 28 for introducing primaryaerosols, the inlets 70 and 72 for the entrainment gas flows from themobility selectors 29 and 42, as well as the suction outlet 62 of thefirst mobility selector 29 and the outlet reserved for the calibratedneutral particles 84. The radioactive source 86 is located at the bottomof the bipolar charging enclosure 62 and the two high voltage supplies90 and 92 are respectively provided for the first mobility selector 29and the second mobility selector 42. There are also annular slots 64, 66and 68 enabling the useful part of the gas flow to pass from one stageto the next. The structure of FIG. 8 is remarkable as a result of itscompactness and ease of use compared with that shown in FIG. 6(references 26 to 32) associating the elementary functions necessary forleading to the same results.

I claim:
 1. A method for calibrating a particle counter comprising thefollowing steps;forming a vector gas flow by an aerosol of particles ofthe same grain size; developing ions in the vector gas with both sign bya bipolar charger, certain particles being electrically charged to astationary charge state in which the distribution of the number ofcharges fixed to each particle follows a Gaussian law (Gunn orBoltzmman); passing the charge aerosol into a mobility selector toattract the charged particles to electrodes therein and classify them asa function of the numbers p of their elementary electrical charges e andallowing said electrically neutral particles to escape; collecting saidneutral particles and passing said neutral particles into said particlecounter to be calibrated, said particles counter displaying a valueN'_(o) ; counting the values of N_(p) and N_(p+1) of said number ofparticles of charges pe and (P+1)e fixed by said selector andcalculating by the formula ##EQU7## the number N_(o) representing thenumber of neutral particles supplied to said counter; and comparingN'_(o) with N_(p).
 2. Apparatus for calibrating a particle counter inwhich particle of the same grain size are produced comprising;acylindrical case; an annular cylindrical bipolar charge space; amobility selector linked with said charge space; said annularcylindrical bipolar charge space and said mobility selector arranged ina superimposed manner within said cylindrical case; said mobilityselector containing electrodes; said annular bipolar cylindrical chargespace receives an aerosol of monodispersed particles; said mobilityselector disperses on said electrodes all said charged particles whichit receives as function their charges; a suction flow of said mobilityselector contains only the neutral particles to be counted.
 3. Apparatusaccording to claim 2 further comprising an electrical mobilitypreselector with circular symmetry.
 4. Apparatus according to claim 2further comprising a primary aerosol injection chamber linked with saidbipolar charge space.
 5. Apparatus according to claim 3 furthercomprising a primary aerosol injection chamber linked with said bipolarcharge space.